/Engine/Source/Runtime/Engine/Classes/GameFramework/GameModeBase.h
Accept or reject a player attempting to join the server. Fails login if you set the ErrorMessage to a non-empty string. PreLogin is called before Login. Significant game time may pass before Login is called
virtual void PreLogin ( const [FString](API\Runtime\Core\Containers\FString) & Options, const [FString](API\Runtime\Core\Containers\FString) & Address, const [FUniqueNetIdRepl](API\Runtime\Engine\GameFramework\FUniqueNetIdRepl) & UniqueId, [FString](API\Runtime\Core\Containers\FString) & ErrorMessage )